1. Field of the Invention
The present invention relates to an optical recording and/or reproducing apparatus, and more particularly, to a high-sensitivity magnetic circuit and an optical recording and/or reproducing apparatus employing the same.
2. Description of the Related Art
In general, an optical recording and/or reproducing apparatus employs an optical pickup to perform non-contact recording and/or reproducing of information on an optical disc, which is an optical information storage medium, while moving in a radial direction of the optical disc.
An optical pickup needs an actuator that drives an objective lens in tracking, focus, and/or tilt directions, so that a beam emitted by a light source is focused to a spot at the proper position on an optical disc. Here, driving the objective lens in a tracking direction means controlling movement of the objective lens in a radial direction of the optical disc, so that a beam spot is positioned at a center of a track.
A typical optical pickup actuator includes a bobbin mounted movably on a base, a suspension that supports the bobbin so that the bobbin is movable with respect to the base, and a pair of magnetic circuits disposed opposite each other on the bobbin and the base, respectively.
An optical pickup actuator basically performs two-axis control in the tracking direction and the focus direction. There is increasing demand for a high-density, small-size, and light-weight optical recording and/or reproducing apparatus.
To achieve high density, an optical pickup actuator needs to perform three- or four-axis control, including tilt control, in addition to two-axis control.
In recent years, a numerical aperture (NA) of an objective lens has increased and the wavelength of a light source has decreased for realization of high-density optical storage devices, which decreases a tilt margin of an optical pickup actuator. To compensate for this, there is a need for an optical pickup actuator designed to perform three- or four-axis control, including tilt control, in addition to tracking and focusing control. Three-axis control means control in focus, tracking, and radial tilt directions, and four-axis control means control in focus, tracking, radial tilt, and tangential tilt directions. Whether the optical pickup actuator can perform two-axis, three-axis, or four-axis control is determined by the structure of a magnetic circuit.
Furthermore, to meet the increasing demand for high speed, there is a need for an optical pickup actuator with improved sensitivity. High-density optical information storage media currently under development, some of which are already commercially available, such as Blu-ray discs (BDs), require an optical pickup actuator with even higher sensitivity than required by CDs or DVDs. For example, a four-speed optical recording and/or reproducing apparatus for BDs needs an optical pickup actuator that is at least as sensitive as an optical pickup actuator of a 16-speed optical recording and/or reproducing apparatus for recordable DVDs.
However, the highest sensitivity available in an optical pickup actuator at present is comparable to that required for 16-speed recordable DVDs. One of the biggest problems encountered in realizing high-speed BDs is that the sensitivity of an optical pickup actuator must also be increased.
Thus, to realize a higher-speed optical recording and/or reproducing apparatus for BDs and/or DVDs, a novel optical pickup actuator with improved sensitivity is highly desirable.
FIG. 4 schematically shows an example of a conventional magnetic circuit that can be used for a 52-speed CD-ROM. FIG. 5 shows another example of a conventional magnetic circuit that can be applied to a 16-speed DVD-ROM. For better visualization, the focus coil is not shown in FIGS. 4 and 5, in which horizontal and vertical directions are tracking and focus directions, respectively.
The conventional magnetic circuit of FIG. 4 includes a unipolar magnet 131 and a pair of tracking coils 140. In this magnetic circuit, only about a fourth of the tracking coils 140 is used for tracking control. It is known that the magnetic circuit constructed as shown in FIG. 4 offers tracking sensitivity of about 40-50 μm/V. Here, V means volt.
The conventional magnetic circuit of FIG. 5 includes a magnet 231 having first and second magnetic portions 233 and 235 magnetized with opposite polarities, and a tracking coil 240 that interacts with the magnet 231. In this magnetic circuit, about a half of the tracking coil 240 is used for tracking control. It is known that the magnetic circuit constructed as shown in FIG. 5 offers tracking sensitivity of about 60-70 μm/V.
As shown in FIG. 5, it is impossible to utilize the tracking coil more efficiently by using more than two sides of the tracking coil 240 for tracking control. Thus, tracking sensitivity can only be improved by increasing the number of tracking coils.